In the production of chemical products, reaction kettles are widely used, in which the reactants are heated by the heating agents put in the jackets on the outside of the kettles or the coil tubes inside the apparatuses. The heating agents are usually high temperature heat-conducting oil or high temperature steam. The heating method is disadvantageous not only in a slow heating speed and low heat efficiency but also unavailable high temperature, which hinders the starting of many new projects for high-efficient reaction kettles for high temperature and highly corrosive reactants. China Patent No. 200720114988.X for “PTFE-insulated heating product” provides a thin-wall cylindrical PTFE heater of integrated structure, which with heating elements imbedded inside its PTFE layer, is placed in a reaction kettle for direct media heating. The product is innovative in eliminating quite a few drawbacks brought by heating by means of jackets or coil tubes. But the patent is still defective in {circle around (1)} inconvenient manufacture, for the product, as an integrated structure, will have to be wholly abandoned (for PTFE cannot be repaired) if a minute damage occurs to the large area of its PTFE layer in the process of manufacture or operation; {circle around (2)} incapacity of being made with a high power, for the product, as a single layer cylindrical structure, has a limited area and cannot contain too many electrical elements, thus hindering its wide application.